Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or another print medium. The ink is driven from the nozzles toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven from a nozzle toward a medium with an electrostatic field. Another ink jet printing procedure, known as squeeze tube, employs a piezo-electric element in the ink nozzle. Electrically-caused distortions of the piezo-electric element pump the ink through the nozzle and toward the print medium. In still another ink jet printing procedure, known as thermal or bubble ink jet printing, the ink is driven from a nozzle toward the print medium by the formation of an expanding vapor-phase bubble in the nozzle. These various printing methods are described in "Output Hard Copy Devices", edited by Durbeck and Sherr, Academic Press, 1988 (see particularly chapter 13 entitled "Ink Jet Printing").
Ink compositions for use in ink jet printers generally comprise deionized water, a water-soluble or water-miscible organic solvent, and a colorant. Generally, the colorant is a soluble dye. Unfortunately, inks comprising soluble dyes can exhibit many problems, such as poor waterfastness, poor lightfastness, clogging of the jetting channels as a result of solvent evaporation and changes in the dye's solubility, dye crystallization, ink bleeding and feathering when prints are formed on plain papers, poor thermal stability, chemical instability, and ease of oxidation. Many of these problems can be minimized by replacing the dyes used in ink formulations with insoluble pigments. In general, pigments have superior properties with respect to dyes, such as good waterfastness, good lightfastness, good image density, thermal stability, oxidative stability, the ability to perform intercolor ink mixing, and compatibility with both coated/treated and plain papers.
In pigmented ink compositions, the pigment dispersion is generally stabilized by a polymeric component. For example, Ma et al., U.S. Pat. No. 5,085,698, disclose an ink composition comprising pigment, aqueous medium, and an acrylic/acrylate block copolymer as a stabilizing agent. Destabilization of the pigment dispersion can result in precipitation of pigment in the nozzle of the ink jet printer which can eventually adversely impact the printing process. Most prior art pigment dispersions will irreversibly clog the nozzle of ink jet printers when left standing in the atmosphere for an extended period of time (e.g., 6 hours).
While prior pigmented ink compositions for ink jet printers did show transient stability for the pigment dispersion, improved pigment ink dispersion is desired to meet the demanding needs of commercial ink jet printers.
It is therefore an object of the present invention to provide an improved pigmented ink composition for ink jet printers.
Other objects and advantages will become apparent from the following disclosure.